Optical repeater integrated lasers

ABSTRACT

Integrated laser diode devices are utilized as repeater elements and logic circuit elements in fiber optic and other light transfer systems. One embodiment discloses a six layer device (10) which is triggered not by an external electrical gating source, but by an external light source (8, 9) as from an optical fiber. Another embodiment operates a laser diode in a bilateral mode. That is, depending on the polarity of the applied voltage bias V to the device (50), two separate light pulses are emitted from different regions (56, 52) of the crystal. A further embodiment utilizes the semiconductor laser as a logical AND function. When the electrical bias (V) of the device (60) is set so that when at least two external light sources (67, 68) are applied, the device will emit laser light (69). Still another embodiment utilizes two semiconductor laser devices (911, 913) as an astable optical multivibrator (90).

This is a division of application Ser. No. 056,765, filed July 12, 1979,now U.S. Pat. No. 4,316,156.

The invention relates to optical repeater integrated lasers; integratedlaser diode devices are utilized as repeater elements and logic circuitelements in fiber optic or other light transfer systems.

BACKGROUND OF THE INVENTION

Semiconductor diode lasers were developed in 1962 almost simultaneouslyby several groups of workers. Since then much development work hasevolved in the development and use of light emitting semiconductordiodes.

There are at least three groups of diode lasers and are classifiedaccording to structure. Simple diode lasers are called homostructurelasers because they are made of a single semiconductor material. Ahomostructure diode laser would comprise, for example, n-type and p-typegallium arsenide (GaAs). The recombination of electrons injected fromthe n-region into the p-region with holes existing in the p-regioncauses the emission of laser light.

In a single-heterostructure semiconductor laser device on additionallayer of aluminum gallium arsenide, (AlGaAs), for example, is added.This type of crystal has had some of the gallium atoms in the galliumarsenide crystal replaced by aluminum atoms. The injected electrons arestopped at the aluminum gallium arsenide layer (junction) resulting in ahigher degree of concentration of light emitted.

In a double-heterostructure semiconductor laser device, for example, alayer of GaAs is sandwiched between two layers of AlGaAs. The barriersset up by the heterostructures cause even further confinement of thelight emitted.

SUMMARY OF THE INVENTION

According to the present invention, several embodiments of semiconductorlaser devices, in the double-heterostructure class, are disclosed. Oneembodiment discloses a six layer p-n-p-n-n-n device which is triggerednot by an external electrical gating source, but by an external lightsource as from an optical fiber. The application of the input lightsource results in a large output light source, so as to function as anoptical repeater.

Another embodiment of the present invention discloses the opticalrepeater aspect thereof but now it can be operated in a bilateral mode.That is, depending on the polarity of the applied voltage bias to thedevice, two separate light pulses are emitted from different regions ofthe crystal. It, therefore, is also selectively responsive to differentimpinging light sources.

A further embodiment of the optical repeater of the present inventiondiscloses the use of the semiconductor diode laser as a logical ANDfunction. That is, the electrical bias of the device is set so that whenat least two external light sources are applied, the device will emitlaser light. Both external sources must be on at the same time to allowthe device to lase. Since one or the other input separately will nottrigger the device, but both sources will, the optical repeater operatesin a logical AND fashion.

Still another embodiment of the present invention utilizes the teachingsof the optical repeater of the present disclosure to generate twooptical pulse trains which alternate in an adjustable repetitionfrequency. Thus, the circuit will operate in an astable opticalmulti-vibrator mode.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference may be hadto the following detailed description of the invention in conjunctionwith the drawings wherein:

FIGS. 1 to 4 show schematic representations of alternate features of oneembodiment of the present invention,

FIG. 5 is a schematc representation of a second embodiment of thepresent invention,

FIGS. 6 to 8 show schematic representations of alternate features ofanother embodiment of the present invention, and

FIG. 9 is a schematic representation of still another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In an article by H. F. Lockwood et al, "The GaAs P-N-P-N Laser Diode",IEEE J. Quantum Electronics, Vol. QE10, No. 7, July 1974, pp. 567-569, ap-n-p-n laser diode is described. It is disclosed therein that the laserdiode was capable of emitting 0.5 watt pulses of approximately 5nanoseconds in duration. It was triggered not by an external gatingsource as is usually the case in p-n-p-n switches, but rather byincreasing the voltage across the entire device until avalanchebreakdown was reached in the reverse biased junction. At this point acapacitor across the device discharged and a short high power lightpulse was emitted. See also U.S. Pat. No. 4,065,729 to Gover et al,issued Dec. 27, 1977, wherein a p-p-p-n-p-n-n laser diode is describedwhich produces coherent laser beams in response to received lightpulses.

In one embodiment of the present invention, there is described analternative way to configure this type of p-n-p-n laser so that light(as from an optical fiber, for example) can be used to trigger thelaser. In this configeration, a small input light pulse will result in alarge output light pulse whose pulse shape is independent of the inputlight pulse shape.

One such device is shown in FIG. 1. It consists of a p-n-p-n structure10 configured as a p-n-p-n-n-n device as follows. Layer 1 is an n-typeGaAs substrate upon which the other layers are grown. Layer 2 is n-typeGa₀.7 Al₀.3 As. Layer 3 is the n-type GaAs lasing region. Layer 4 is thep-type Ga₀.7 Al₀.3 As laser region. Layer 5 is the n-type Ga₀.9 Al₀.1 Asbase region. Layer 6 is the p-type Ga₀.7 Al₀.3 As layer. A forward biasvoltage V is placed across the entire structure by ohmic contacts 7, thevoltage V being slightly less than the breakdown voltage of the backbiased junction between layers 4 and 5. When light is supplied by lightsources 8 or 9 and is absorbed in one of the two base regions (4 or 5),electron hole pairs will be generated which allow the device to switchinto the current conducting state. That is, the reverse biased junctionbetween layers 4 and 5 will become forward biased and holes will beinjected from region 6 into the n-type GaAs region 3 resulting in laseremission from region 3.

The bandgap of the input light sensitive material must always be smallenough to absorb the incident photons. Thus, if it is desired to have arepeater which emitted the same wavelength as the trigger lightwavelength, the laser radiator active region should have more aluminumthan the receiving region. For example, in FIG. 1 layer 5 would ben-type GaAs; and layer 3 would be Ga₀.95 Al₀.05 As, with the otherlayers remaining the same.

Referring now to FIG. 2, if a constant voltage V is applied to thestructure 10, the current will be constant and will be determined by theseries resistance 11 of the device 10 and the external circuit. Thelaser will continue to operate unless the current or voltage drops belowthe "holding" current or "holding" voltage for device 10. A drop in theholding current is achieved if the laser device 10 is biased as shown inFIG. 2. In this case the laser will emit a pulse of light the durationof which is dependent on the external circuit parameters. The on time ofthe laser pulse T_(l) can be expressed as: ##EQU1## where I_(th) is thethreshold lasing current; R_(s) is the series resistance in the laserbranch including the internal resistance of the laser 10, R_(si), plusthe external resistance R_(so) ; C is the external capacitor value and Vis the applied voltage.

FIGS. 3 and 4 depict other configurations which operate in accordancewith the principles of the present invention. FIG. 3, for example,discloses a laser device 30 wherein layer 31 is n-type Ga₀.7 Al₀.3 As;layer 32 is n-type GaAs; layer 33 is p-type Ga₀.7 Al₀.3 As; layer 34 isn-type GaAs; while layer 35 is p-type Ga₀.7 Al0.3As. When incoming lightimpinges on layer 35, the light will be emitted from layer 32. FIG. 4discloses another device 40 within the principles of the presentinvention. In this device, layer 41 is n-type Ga₀.7 Al₀.3 As; layer 42is p-type GaAs, layer 43 is n-type GaAs; while layer 44 is Ga₀.7 Al₀.3As. When incoming light impinges on layer 44, the laser light outputsfrom layer 42.

There are other configurations that also work in accordance with theprinciples of the present invention. For example, an n-p-n-p device willfunction in the same manner. Also, variations in the aluminum (Al)content of any of the layers will not afffect operations except that thelasing region should be a low bandgap region in the device and lightmust be absorbed in one of the two base regions to trigger the device.The light emitting layer might also be p-type in the device shown inFIG. 1 and could be moved to between layers 5 and 6. In this case, thetrigger light pulse 8 would have to impinge on the bottom of the deviceso it would not be absorbed in the light emitting layer. The devices setforth in FIGS. 1 to 4 disclose improved integrated optical repeaterswherein light is amplified and repeated directly in the same crystal.

FIG. 5 discloses a bilateral p-n-p-n switch 50 which can emit thesepulses with one of two spatially separated optical fibers depending onthe polarity of the bias applied to the device. Thus it is not necessaryto use an optical deflector to spatially scan the beam. In addition,device 50 is capable of determining which of two spatial locations (thatis, of two input optical fibers) the input light pulse was emitted from.

Laser diode 50 in FIG. 5 is comprised of two p-n-p-n type switches, oneof which is forward biased while the other is reversed biased. Layer 51is p-type Ga₀.7 Al₀.3 As; layer 52 is p-type GaAs; layer 53 is a n-typeGa₀.7 Al₀.3 As; layer 54 is p-type Ga₀.9 Al₀.1 As; layer 55 is n-typeGa₀.7 Al₀.3 As; layer 56 is p-type GaAs; while layer 57 is Ga₀.7 Al₀.3As. If light is absorbed in base layer 55 while the switch 58 is in theA position, the junction of layers 56 and 55 will be forwarded biasedand the device will switch on and begin to lase with light emitter fromlayer 56. The junction of layers 52 and 53 will be back impinged onlayer 53. When the switch 58 is thrown to the B position, the junctionbiases are reversed and light will be emitted from layer 52 but notlayer 56. If an external RC circuit is provided for the device 50 inFIG. 5 similar to that shown in FIG. 2, the device 50 will emit a shortpulse of light from either the layer 52 or 56 depending on the positionsof switch 58. Thus, depending on the position of switch 58, light willbe selectively emitted from layers 52 or 56 or selectively activated bylight impinged on layer 53 or 55. If the second switch of the laserlases at a different wavelength than does the first one, frequencymultiplexing into the same or different optical fibers can be achieved.

Extending the principle of the present invention wherein the laserdevice is impinged with two independent light sources, attention is nowdirected to FIG. 6. This figure describes a device 60 in which twosimultaneous input light pulses 67 and 68 are used to stimulate thegeneration of a single output pulse 69. Such a device could function asa logic element in an integrated optical circuit or optical repeatercircuit. As an alternative, the device in FIG. 6, and FIGS. 7 and 8 forthat matter, could be utilized to produce a single output pulse uponsimultaneous electrical inputs instead of the simultaneous input lightpulses. Further, an amplified electrical current can similarly begenerated.

FIG. 6 shows one such laser device 60. It consists of layers of sixalternating conductivity types, i.e., p-n-p-n-p-n or n-p-n-p-n-p. Layer61 is n-type Ga₀.7 Al₀.3 As; layer 62 is p-type GaAs; layer 63 is n-typeGa₀.7 Al₀.3 As; layer 64 is p-type Ga₀.7 Al₀.3 As; layer 65 is n-typeGaAs, and layer 66 is p-type Ga₀.7 Al₀.3 As. There are two reversebiased junctions (64, 65 and 62, 63) when a forward bias V is appliedacross the device as shown in FIG. 6. Light sources 67 and 68 (and/orelectrical contacts) are used to create minority carriers in thevicinity of the two back biased junctions. When device 60 is biased nearthe breakover voltage of the back biased junctions, injection of theseminority carriers causes the device to switch into the currentconducting state and the device begins to lase as shown as laser output69. Thus, in operation as an optical AND function, both light sources 67and 68 (or equivalent electrical source contacts) must be on at the sametime to cause light output pulse 69 to be emitted.

The laser operation of the device 60 in FIG. 6 will continue until thevoltage V across or the current through the device drops below the"holding" voltage or "holding" current. This aspect can be establishedby placing the device 60 in an RC circuit similar to the one shown anddescribed in conjunction with FIG. 2. This circuit allows the pulsewidth of the amplified pulse to be varied by changing the resistor andcapacitor values.

FIG. 7 shows an alternate device 70 for the optical AND function fromthat set forth in FIG. 6. Layer 71 is n-type Ga₀.7 Al₀.3 As; layer 72 isp-type GaAs; layer 73 is n-type Ga₀.7 Al₀.3 As; layer 74 is n-type GaAs;layer 75 is p-type Ga₀.7 Al₀.3 As; layer 76 is n-type GaAs; and layer 77is p-type Ga₀.4 Al₀.6 As. Again, in order to produce light output 791,light inputs (or equivalent electrical source contacts) 78 and 79 mustbe present simultaneously to produce the optical AND function operation.While in FIG. 6, the light source 67 impinges on layer 65 and lightsource 68 impinges on layer 62 to produce the laser output 69 from layer62; in FIG. 7, light source 78 impinges on layer 76 and light source 79impinges on layer 72 to generate the laser light output 791 from layer74.

FIG. 8 shows another alternate device 80 for the optical AND functionfrom that set forth in FIGS. 6 and 7. Layer 81 is n-type Ga₀.6 Al₀.4 As;layer 82 is p-type GaAs; layer 83 is n-type Ga₀.7 Al₀.3 As; layer 84 isp-type GaAs; layer 85 is p-type Ga₀.7 Al₀.3 As; layer 86 is n-type GaAs;and layer 87 is p-type Ga₀.7 Al₀.3 As. In order to produce layer lightoutput 891, light inputs 88, 89 (or equivalent electrical sourcecontacts) must be present simultaneously to produce the optical ANDfunction. In FIG. 8, light output 891 emanates from layer 84 when lightsource 88 impinges on layer 86 and light source 89 impinges on layer 82.

The key criteria for operation of the laser devices shown and describedin conjunction with FIGS. 6, 7, and 8, are: (1) the input light musthave a wavelength such that it is absorbed in the region of the backbiased junctions; (2) if laser output is desired, the lasing region mustbe surrounded by wider bandgap GaAlAs to achieve current and opticalconfinement; and (3) the lasing region should have a bandgap which is aslow as or slightly lower than any other layer in the crystal so thatgood injection efficiency is obtained.

Extending the invention to another embodiment, FIG. 9 utilizes two diodelasers as set forth above, but now employed in an astable opticalmultivibrator configuration. The multivibrator 90 will generate twooptical pulse trains 901, 903 which alternate. The repetition frequencyis adjustable as seen below.

Both diode lasers 905 and 907 are p-n-p-n devices as set forth in FIG.1, for example. Further, the circuit shown in FIG. 9 is connected to a+30 V supply and the components have the values shown, however, theelectro-optic circuit is just an example, the circuit will function withother element values as well.

If neither laser 905, 907 is emitting light, the overall electro-opticcircuit is quiescent, V1, V2, and V3 are all zero, and V₁ =V_(b) =30 Vwhich is assumed to be below the breakdown voltage of the back biasedjunction 905-2/905-3 of laser 905 and 907-2/907-3 of laser 907. If asmall amount of external light is now used to illuminate layer 905-3 or907-3 (layer 905-3 in FIG. 9), the circuit 90 begins to function asfollows. Diode laser 905 switches on and voltage Va drops to about 1.5volts. Thus a very large current flows through capacitor 911, laserdiode 905 and resistor 917. The voltage V3 is very large at this point,about 28.5 volts, but decays with a time constant of approximately(R917)(C911)=15×10×10⁻¹² =150×10⁻¹² seconds toward an equilibrium valueof 1.5 volts. At that time V1 would be approximately 27 volts.

Throughout this time diode laser 905 is emitting light and a smallamount of its output light 903 is allowed to impinge on layer 907-3. Ascapacitor 911 charges, V3 falls toward 1.5 volts and V_(b), therefore,increases from an initial value of approximately 1.5 volts toward 28.5volts. Before this latter voltage is achieved, however, the combinationof light impinging on layer 907-3 and increasing voltage causes laserdiode 907 to switch ON. At the instant this occurs, there is a surge ofcurrent through capacitor 915, laser diode 907 and resistor 917 whichsubstantially increases voltage V3 and switches OFF laser diode 905. Nowcapacitor 911 discharges through resistor 909 with a time constant of(R909)(C911)=270×10×10⁻¹² =2.7×10⁻⁹ seconds. Light 901 from laser diode907 illuminates layer 905-3 and when voltage V_(a) increasessufficiently, diode 905 switches ON and diode 907 switches OFF. Thisprocess repeats indefinitely until some event occurs to interrupt it,such as momentarily setting the power supply voltage to zero or removingthe external light source.

The repetition frequency of the multivibrator action is adjustable byvarying the component values; for example, resistor 909/capacitor 911and/or resistor 913/capacitor 915. The pulses generated by laser diodes905, 907 need not be equal in duration: (R909)(C911) can differ from(R913)(C915).

While the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made without departingfrom the essential teachings of the invention. For example, the lasershave been described with the percentages of gallium, aluminum andarsenide being Ga₀.7 Al₀.3 As. The lases will also operate in accordancewith the principle of the invention with percentages between Ga₀.85Al₀.15 and Ga₀.4 Al₀.6. Further, other semiconductive combinations maybe utilized, such as, Ga₁₋₄ In_(x) As_(1-y) Py.

What is claimed is:
 1. A bilateral switch comprising a plural layersemiconductor diode laser which is electrically biased to the pointwherein light input causes the laser to commence lasing from an activesemiconductor layer, such that the improvement is characterized by:afirst plural layer diode laser switch of intermixed layers of n and ptype semiconductor material, the junction of two of said intermixedlayers being forward biased at the application of the electrical bias ofone polarity such that laser light is emitted from a first activesemiconductor layer, a second plural layer diode laser switch on thesame crystalline structure as said first laser switch of intermixedlayers of n and p type semiconductor material, the junction of twodifferent intermixed layers being reverse biased at the application ofsaid electrical bias of one polarity, wherein said first laser switch isreverse biased at the application of an electrical bias of a secondpolarity and said second laser switch is forward biased at theapplication of said electrical bias of a second polarity, such thatlaser light is emitted from a second active semiconductor layer.
 2. Thebilateral switch as set forth in claim 1 wherein certain of saidintermixed layers of said first diode laser switch are shared with andare the same as certain of said intermixed layers of said second diodelaser switch in the same crystalline structure of said diode laser. 3.The bilateral switch as set forth in claim 2 wherein light impinged onone or the other of said first and second diode laser switches willcause laser light to be selectively emitted from said first or seconddiode laser switches depending on the polarity of the externalelectrical bias applied to the entire crystalline structure.
 4. Thebilateral switch as set forth in claim 3 wherein the frequencies oflight emitted from said first or second diode laser switches aredetermined by the composition of material comprising the intermixedlayers of semiconductor material of the entire crystalline structure. 5.The bilateral switch as set forth in claim 2 wherein light impinged onsaid first diode laser switch will cause laser light to be emitted fromsaid first diode laser switch upon application to the crystallinestructure of an electrical bias of one polarity and, upon the switchingof said electrical bias to the second polarity, will cause laser lightto be emitted from said second diode laser switch.
 6. The bilateralswitch as set forth in claim 5 wherein light impinged on said seconddiode laser switch will cause laser light to be emitted from said seconddiode laser switch upon application to the crystalline structural ofsaid electrical bias of second polarity and, upon the switching of saidelectrical bias to the other polarity, will cause laser light to beemitted from said first diode laser switch.
 7. The bilateral switch asset forth in claim 2 wherein said crystalline structure of intermixedlayers comprises:a first layer (51) of p-type Ga₀.7 Al₀.3 As, a secondlayer (52) of p-type GaAs, a third layer (53) of n-type Ga₀.7 Al₀.3 As,a fourth layer (54) of p-type Ga₀.9 Al₀.1 As, a fifth layer (55) ofn-type Ga₀.7 Al₀.3 As, a sixth layer (56) of p-type GaAs, and a seventhlayer (57) of p-type Ga₀.7 Al₀.3 As.
 8. The bilateral switch as setforth in claim 7 wherein the light is selectively emitted from thesecond or sixth layers.
 9. The diode laser as set forth in claim 2, 3 or4 and further including:a constant voltage bias supply, a capacitor inparallel to and connected to said bias supply and said diodelaser,wherein at the impingement of either of said external lightsources, said diode laser will emit a pulse of light the duration ofwhich can be expressed by the relationship ##EQU2## where T₁ is theduration of said light pulse, R_(s) is the series resistance of saidlaser diode branch, C is the value of the external capacitor, I_(th) isthe threshold lasing current, V is the value of the applied voltage.